The present invention relates to a thin film bulk acoustic wave resonator and the production method of the same.
A film bulk acoustic wave resonator (FBAR) excelling in a filter characteristic and making it possible to a miniaturization of the RF front-end module receives an attention as an element used in a circuit such as a transmission filter and a duplexer in a wireless communication system such as a mobile phone and a wireless sensing system.
As a structure and a production method of the above-mentioned FBAR, methods such as (1) a substrate topside-depression method, (2) a substrate backside-via-hole method, (3) an air bridge method, (4) a multilayer acoustic mirror method are known.
In the above, an air bridge method among the above-mentioned methods is known as one of a method having high possibility of a cost-reduction.
For example, there is a description that an air bridge can be formed with using a sacrificial layer composed of zinc oxide (ZnO) in a production method of an air bridge structure FBAR in an article entitled “An Air-Gap Type Piezoelectric Composite Thin Film Resonator”, IEEE Proc. 39th Annual Symp. Freq. Control, pp. 361–355 (1985), by Hiroshi Satoh, Yasuo Ebata, Hitoshi Suzuki and Choji Narahara.
Further, there is a description that an FBAR having effective characteristics can be produced by adopting copper (Cu) for a sacrificial layer in a production method of an air bridge structure FBAR in an article entitled “Thin Film Bulk Acoustic WaveResonator”, IEEE Transaction on Ultrasonics, Ferroelectrics and Frequency Control (2002).
Further, it is described about a method improved for forming an FBAR and an FBAR formed on a substrate having more effective characteristics than a substrate used conventionally in Kohyo, Jpn. Unexamined Patent Publication No. 2002-509644.
On the contrary, it is described about a production method able to mount an FBAR on a semiconductor integrated circuit more effectively than related prior art by using a sacrificial layer composed of germanium (Ge) to form an air bridge in an article entitled “Aluminium Nitride Thin Film 2 Gz Resonator Using Germanium Sacrificial Layer Etching”, MEMS Symposium 2002 Tohoku Univ, by Masayoshi Esashi and Motoaki Hara.
FIG. 11 is a cross sectional view showing composition of the above-mentioned air bridge structure FBAR according to a conventional example.
A bottom electrode 101, a piezoelectric film 102 and a top electrode 103 are laminated on a substrate 100.
Here, a predetermined gap G is placed between a bottom electrode 101 and a substrate 100 and it becomes a cavity as a resonance region enabling an oscillation of the piezoelectric film.
In the above-mentioned an air bridge structure FBAR, a convex surface ups and downs due to the gap G becoming a resonance region is characterized structurally. Consequently, a step shape due to a convex surface ups and downs is formed in a portion corresponding to a circumference edge Sa of the gap G for the piezoelectric film 102, and an accumulation and a concentration of an excessive film stress are arisen in the piezoelectric film 102.
Further, as a piezoelectric film formed in an FBAR, for example, it is desirable to adopt a ceramic piezoelectric material having a superior piezoelectric constant and an elastic constant to a bulk elastic device such as AIN, ZnO and PZT. However, these materials have remarkably high brittleness generally. Consequently, it includes a problem that a crack of a piezoelectric film by an accumulation and a concentration of a local stress to a piezoelectric film is raised easily.
A partial stress concentration to the ceramic piezoelectric films begins to raise a crack increasingly with adding an effect such as a physical impact, a heat treatment cycle and an interaction with an added piezoelectric film stress.
Environment loads of these slight processes induce defects of a piezoelectric film and those defects are clarified increasingly in subsequent processes.
On the contrary, a piezoelectric film having superior piezoelectric characteristics and elastic characteristics is desired for improving resonance characteristics of an FBAR. Now forming a film is performed with a sputtering technique usually to obtain those piezoelectric films, and it is necessary to obtain a dense film highly oriented to a C axis particularly in the case of AIN, ZnO and the like.
An internal stress of the film tends to become high as a film fills such characteristics. As a result, the high internal stress foments further a crack in a surface ups and downs portion in a region around a gap of an air bridge structure FBAR.
Further, mixed loading onto a semiconductor integrated circuit is the most advantageous composition because of compliance of a production process about an air bridge structure FBAR. However, a large restriction in mixed-loading on a semiconductor integrated circuit is a low-temperature film deposition system (e.g., a sputtering process in 400° C. or less) of a piezoelectric film.
This low-temperature sputtering process is similar to the above, an internal stress of a piezoelectric film tends to become high. As a result, the high internal stress foments further a crack in a surface ups and downs portion in a region around a gap of an air bridge structure FBAR.
Therefore, there is a large problem to realizing a provision of an FBAR having stable composition not causing a crack of a piezoelectric film, improvement of productivity and a cost-reduction due to heightening yield and more superior resonance characteristics (highly orientation and densification of a piezoelectric film) and relaxing a local stress concentration of this piezoelectric film for realizing a mixed loading process onto a semiconductor integrated circuit of such an FBAR.